Salts comprising an organic, inorganic or organometallic cation and a nonnucleophilic counteranion have been shown to have utility as photochemically and thermally activated initiators for cationic addition polymerization or as similarly activated latent catalysts for step-growth (or condensation) polymerization, depolymerization and unblocking of functionalized polymers. Common commercial photoinitiator salts include onium and organometallic salts such as diaryliodonium and triarylsulfonium salts and (cyclopentadienyl)(arene)iron.sup.+ salts of the anions PF.sub.6.sup.- and SbF.sub.6.sup.-. In certain cases, these same salts may also photoinitiate free-radical addition polymerization and are useful in "dual cure" applications where a mixture of cationically sensitive and free-radically polymerizable monomers are polymerized either simultaneously or sequentially.
For many commercial applications, the polymerizable monomers are multifunctional (i.e., contain more than one polymerizable group per molecule), for example, epoxides, such as diglycidyl ether of bisphenol A (DGEBA) and vinyl ethers, such as 1,4-cyclohexanedimethanol divinyl ether (CHVE). Mixtures of multifunctional monomers such as isocyanates and alcohols or epoxides and alcohols can undergo catalyzed polycondensation via a step-growth mechanism.
Photochemically activated initiators (or catalysts) typically provide homogeneous mixing of the monomers with the initiator prior to polymerization, or selective activation by light for imaging applications, such as photolithography. Simple photoinitiators typically have low absorption coefficients above 300 nm where a major portion of the spectral output of conventional light sources (i.e., medium and high pressure mercury lamps, fluorescent lamps or the sun) occurs. These low absorption coefficients tend to limit their photoefficiency.
To overcome this problem, a number of methods have been developed to improve the wavelength response of such photoinitiators. For example, photosensitizers may be added in combination with the photoinitators to more efficiently transfer light energy to the cationic portion of the initiator. In addition synthetic modifications of the cationic portions of onium or organometallic photoinitiator salts can improve photoefficiency.
Synthetic modifications of the cationic portion have been made to improve the solubility of cationic photoinitiators, which, because of their ionic nature, tend to exhibit poor solubility in organic monomers. However, the difficulty and cost of introducing solubilizing substituents has limited commercial application of these materials. Alternative solutions that use reactive diluents or solid dispersants have also been disclosed.
In many applications photoinduced polymerization is impossible, impractical or undesirable. For example, in the many situations where the polymerization reaction occurs in a closed environment (i.e., in a mold or in a laminated product) or where the polymerizable composition may contain opacifying pigments, thermally activated initiators are preferred. The thermally-activated initiators, such as art known onium or organometallic salts may initiate polymerization at ambient or higher temperatures depending upon the specific application. Additional additives, such as oxidants, reductants, metal salts, organic acids or anhydrides, and mixtures thereof are frequently added to control the temperature at which cationic polymerization will occur.
In addition to the art known onium or organometallic salts, acid salts of various amines, metal salts of fluoroalkanesulfonic acids and bis(fluoroalkylsulfonyl)methanes have been used as thermal initiators for cationic addition polymerization of vinyl ethers and epoxies or catalysts for alcohol-epoxy step-growth polymerization.
A key feature of initiators for cationic addition polymerization is the ability to produce powerful Bronsted or Lewis acids upon thermal or photochemical activation. In order to achieve this, counteranions which are nonbasic, nonnucleophilic and nonreducing and therefore stable in a highly acidic and oxidizing environment are essential to prevent initiator deactivation or cationic chain termination. For this reason most cationic initiators used in cationic addition polymerization are based on the anions SbF.sub.6.sup.-, AsF.sub.6.sup.-, PF.sub.6.sup.-, and BF.sub.4.sup.-.
It is known the nature of the counteranion in a complex salt can influence the rate and extent of cationic addition polymerization. See for example, J. V. Crivello, R. Narayan, Chem. Mater., 4, 692, (1992), discussing the order of reactivity among commonly used nonnucleophilic anions is SbF.sub.6.sup.- &gt;AsF.sub.6.sup.- &gt;PF.sub.6.sup.- &gt;BF.sub.4.sup.-. The influence of the anion on reactivity has been ascribed to three principle factors: 1) the strength of the protonic or Lewis acid generated, 2) the degree of ion-pair separation in the propagating cationic chain and 3) the susceptibility of the anions to fluoride abstraction and consequent chain termination. Evidence indicates that, for onium salts, the choice of anion has no effect on the photoefficiency of active acid production.
The use of tetrakis 3,5-bis(trifluoromethyl)phenyl!borate (TFPB.sup.-) and related F- or CF.sub.3 -substituted tetraphenyl borates as highly lipophilic and chemically stable, anions for the extraction of alkali metal cations into organic solvents is described by H. Kobayashi, et. al. in Bull. Chem. Soc. Jpn., 57, 2600 (1984) and Kenkyu Hokoku-Asahi Garasu Kogyo Gijutsu Shoreikai, 42, 137-45, (1983). The further utility of Na TFPB! anion as a phase transfer catalyst to promote electrophilic reactions (e.g. diazo coupling, Friedel-Crafts alkylation, nitrosation) in two phase, aqueous-organic systems is disclosed by H. Kobayashi, et. al. in Yuki Gosei Kagaku Kyokaishi, 46 (10), 943-54 (1988); Chemistry Letters, 579-580, 1981; ibid, 1185-6 (1982); Bull. Chem. Soc. Jpn., 56, 796-801 (1983) and Tetrahedron Lett., 24 (32), 4703-6 (1983). The accelerating or catalytic effect of TFPB.sup.- in these cases has been attributed to the "dehydration" (removal of solvated water) of the reactive cationic species upon transport into the nonpolar organic phase.
Photochromic salts of 4,4'-bipyridinium ions with TFPB.sup.- counteranion which undergo persistent and reversible color changes due to photoinduced electron transfer upon excitation of an ion-pair charge-transfer band have been described by T. Nagamura, et al. in Ber. Bunsen-Ges. Phys. Chem., 93 (12), 1432-6 (1989); ibid, 92 (6), 707-10 (1988); J. Chem. Soc. Chem. Commun., 72-74, (1991) and J. Chem. Soc., Faraday Trans. 1, 84 (10), 3529-37, (1988).
K. R. Mann, W. M. Lamanna and M. G. Hill in Inorg. Chem., 30, 4687 (1991) describe the use of tetrabutylammonium TFPB! as a convenient noncoordinating electrolyte for electrochemical studies in methylene chloride solution.
M. Brookhart, et. al., Polymer Preprints, 32 (1), 461 (1991) and U.S. patent application No. 07/513,241 filed Apr. 20, 1990 describes TFPB.sup.- salts of organometallic metal-alkyl or metal-hydride cations which are useful catalysts for Ziegler-Natta type polymerization of ethylene and higher olefins. The TFPB.sup.- anion provides improved catalyst stability relative to analogous BF.sub.4.sup.- salts. In related work, M. Brookhart, S. Sabo-Etienne, J. Am. Chem. Soc., 113, 2777-79 (1991) describes a TFPB.sup.- salt of an organometallic rhodium-hydride cation, which is an efficient catalyst for the tail-to-tail dimerization of methyl acrylate.
M. Bochmann, A. J. Jaggar, J. Organometal. Chem., 424, C5-7 (1992) describes cationic titanium-alkyl salts of TFPB.sup.- which are active catalysts for the Ziegler-Natta type polymerization of ethylene. The TFPB.sup.- anion provides improved catalyst activity and solubility compared to simple tetraphenylborate. Other cationic metal-alkyl catalysts have been described which utilize fluorine substituted aryl borate counteranions, most notably (C.sub.6 F.sub.5).sub.4 B.sup.-, (CH.sub.3)(C.sub.6 F.sub.5).sub.3 B.sup.-. The catalysts are useful for the Ziegler-Natta polymerization of hydrocarbon olefins such as ethylene and propylene.
J. V. Crivello, J. H. W. Lam, J. Polym. Sci., Polym. Lett. Ed., 17(12), 759 (1979), describe the reactivity of triphenylsulfonium tetraphenylborate as a photoinitiator. Although active for free-radical polymerization, the tetraphenylborate salt was found to be completely inactive for cationic addition polymerization of cyclohexene oxide, a particularly reactive epoxy monomer. The absence of epoxy reactivity contrasts with the high level of epoxy-curing activity displayed by the corresponding triphenylsulfonium SbF.sub.6 ! salt. The difference is attributed to the relatively nucleophilic or basic character of the tetraphenylborate anion.